linux-kernel - Re: [PATCH v3 3/3] exec: do unshare_files after de

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Date:   Sun, 16 Sep 2018 18:59:14 +0200
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Jeff Layton <jlayton@...nel.org>
Cc:     viro@...iv.linux.org.uk, berrange@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] exec: do unshare_files after de_thread

Jeff Layton <jlayton@...nel.org> writes:

> POSIX mandates that open fds and their associated file locks should be
> preserved across an execve. This works, unless the process is
> multithreaded at the time that execve is called.
>
> In that case, we'll end up unsharing the files_struct but the locks will
> still have their fl_owner set to the address of the old one. Eventually,
> when the other threads die and the last reference to the old
> files_struct is put, any POSIX locks get torn down since it looks like
> a close occurred on them.
>
> The result is that all of your open files will be intact with none of
> the locks you held before execve. The simple answer to this is "use OFD
> locks", but this is a nasty surprise and it violates the spec.
>
> Fix this by doing unshare_files later during exec, after we've already
> killed off the other threads in the process. This helps ensure that we
> only unshare the files_struct during exec when it is truly shared with
> other processes.
>
> Note that because the unshare_files call is now done just after
> de_thread, we need a mechanism to pass the displaced files_struct back
> up to __do_execve_file. This is done via a new displaced_files field
> inside the linux_binprm.

Actually because unshare_files happens after de_thread (the point of no
return) we don't need to do anything with displaced files.  A failing
exec will clear brpm->mm, and search_binary_handler will convert any
error into force_sigsegv.

> Cc: Eric W. Biederman <ebiederm@...ssion.com>
> Reported-by: Daniel P. Berrangé <berrange@...hat.com>
> Signed-off-by: Jeff Layton <jlayton@...nel.org>
> ---
>  fs/exec.c               | 11 ++++++-----
>  include/linux/binfmts.h |  1 +
>  2 files changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index c8a68481d7eb..b4a7e659d908 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1262,6 +1262,10 @@ int flush_old_exec(struct linux_binprm * bprm)
>  	if (retval)
>  		goto out;
>  
> +	retval = unshare_files(&bprm->displaced_files);
> +	if (retval)
> +		goto out;
> +

We can place this just above do_close_on_exec.  And simply modify
unshare_files to instead of returning displaced simply put it.

We gain nothing be returning displaced_files as we are already past
the point of no return, and userspace will get an unblockable SIGSEGV
and exit.

>  	/*
>  	 * Must be called _before_ exec_mmap() as bprm->mm is
>  	 * not visibile until then. This also enables the update
> @@ -1713,7 +1717,7 @@ static int __do_execve_file(int fd, struct filename *filename,
>  {
>  	char *pathbuf = NULL;
>  	struct linux_binprm *bprm;
> -	struct files_struct *displaced;
> +	struct files_struct *displaced = NULL;
>  	int retval;
>  
>  	if (IS_ERR(filename))
> @@ -1735,10 +1739,6 @@ static int __do_execve_file(int fd, struct filename *filename,
>  	 * further execve() calls fail. */
>  	current->flags &= ~PF_NPROC_EXCEEDED;
>  
> -	retval = unshare_files(&displaced);
> -	if (retval)
> -		goto out_ret;
> -
>  	retval = -ENOMEM;
>  	bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
>  	if (!bprm)
> @@ -1817,6 +1817,7 @@ static int __do_execve_file(int fd, struct filename *filename,
>  	would_dump(bprm, bprm->file);
>  
>  	retval = exec_binprm(bprm);
> +	displaced = bprm->displaced_files;
>  	if (retval < 0)
>  		goto out;
>  
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index c05f24fac4f6..d7ec384bb1b0 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -49,6 +49,7 @@ struct linux_binprm {
>  	unsigned int taso:1;
>  #endif
>  	unsigned int recursion_depth; /* only for search_binary_handler() */
> +	struct files_struct * displaced_files;
>  	struct file * file;
>  	struct cred *cred;	/* new credentials */
>  	int unsafe;		/* how unsafe this exec is (mask of LSM_UNSAFE_*) */